[ubuntu] low disc space for ubuntu

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http://ubuntuforums.org – i am using ubuntu12.04. i have a 320 gb hard disc. partitioned 275-32-13. i use 13gb for ubuntu. 32for windows7.275for my common files like videos,songs etc. now my system shows low disc space in ubuntu drive .also say to move unused programes to other drive. i am not a computer expert. i want to know which one is unused. is there any application to find this out?. is there any way to increase my drive space. i have enough space in 275 gb file system. (Hardware)